4i6 SECTIONAL TRANSACTIONS.— C. 



Sunday, August 21. 



Excursion to the Brecklands. 



Monday, August 22. 



Presidential Address by Prof. H. H. Swinnerton on Development and 

 evolution (lo.o). (See p. 57.) 



Discussion on The origin of carbonate rocks associated with alkali-rich 

 intrusions (11. 15). 



Dr. H. VON ECKERMANN. 



Daly mentions the Alno alkahne area as clear evidence of carbonate- 

 syntexis. A new survey, however, has proved the existence of a confocal 

 stereometrical distribution both of the carbonatites and the alkaline rocks 

 in general. The emplacement of the rocks is similar to that of Julianehaab 

 and Umptek, as emphasised by Backlund. The carbonatites of Alno, 

 consequently, are not metamorphic limestone-xenoliths. Nor is such a 

 syntexis supported by mineralogical and chemical evidence or by our 

 present knowledge of the Fennoscandian rock-ground. 



Surrounding the Alno-neck, carbonatitic (calcitic or dolomitic) cone- 

 sheets, converging towards the apex of a diatreme, are cut by vertical, 

 radiating alnoitic dykes corresponding to the damkjernites of Fen and 

 representing iron-rich magma risen from below on the blowing out of the 

 diatreme. A gravitational magmatic differentiation accompanied by rising 

 COz-concentration is suggested. The CO2 seems to have greatly affected 

 the normal equilibria of the rock components. 



The discovery of carbonate-bearing differentiates of alkaline character, 

 associated with basic Jotnian magmas, suggests a differential relationship 

 between the latter and the alkaline rocks of Fennoscandia. The incongruent 

 melting of orthoclase, previously emphasised by Bo wen, may be reconsidered 

 from a new angle, involving the Jotnian tensional earthcrust-stresses and 

 the presence of CO 2. 



The Fennoscandian alkaline intrusions — except the post-Caledonian 

 Seiland dykes — are all of the same pre-Cambrian and late- or post- Jotnian 

 age. Whether they are associated with carbonatites or not, they owe their 

 birth to the same fundamental principles. The accumulated evidence of 

 the last few years justifies the draft of a tentative common petrogenetic 

 scheme, which may serve as a basis of further discussion, 



Lt.-Col. W. Campbell Smith. — Alkali-rocks associated with limestones 

 of apparently intrusive stature in southern Nyasaland. 



In the neighbourhood of Lake Chilwa and elsewhere in southern Nyasa- 

 land some alkah-rocks occur in and about a number of vents of supposedly 

 volcanic origin filled with brecciated feldspar-rock intimately associated 

 with crystaUine limestones. The field-occurrence of these rocks has been 

 fully described by Dr. F. Dixey of the Geological Survey of Nyasaland. 

 The crystalline limestones of the Basement Complex in the district are of 

 negligible bulk as compared with those of the volcanic vents and are different 

 in composition. The feldspathic breccias of many of the vents consist of 

 angular fragments of orthoclase-rock with unusually high potash content. 



